The tonotopic map of the avian basilar papilla (Callus domesticus) will be identified for two age groups (E16-17 and 2 weeks post hatch). This will be undertaken to test the hypothesis that the tonotopic map changes substantially throughout development and is not simply a "scale model" of the adult. To accomplish this, individual eighth nerve neurons will be impaled. The characteristic frequency and spontaneous rate will be determined. Horseradish-peroxidase (HRP) will be iontophoretically injected into the cell and allowed to distribute throughout the cell as a cell tag. Terminals in the basilar papilla will then be identified histologically and their position expressed as a percent of the total length and width of the basilar papilla.